1. Field of the Invention
This invention relates to novel polyamides and to polymerization processes for producing them. More particularly, this invention relates to polyamides having novel end groups and to novel initiator compounds that are effective, in anionic polymerization processes, to cause the polymerization process to take place at rapid rates and at comparatively low temperatures despite the presence of end groups that could otherwise hinder or stop the polymerization.
2. Description of the Prior Art
The polymerization of lactams using anionic polymerization processes (as defined more fully below) has become a commercially significant method for preparing shaped lactam articles. Since the polymerizations can be conducted at temperatures below the melting point of the resulting polylactam, the liquid phase of the polylactam is avoided and the preparation of massive polylactam shapes in inexpensive molds and at atmospheric pressures is facilitated. In addition to providing rapid polymerizations and direct conversion from liquid monomer to solid polymer, these polymerization processes are advantageous in that the product generally has a low monomer content. Since the equilibrium constant favors monomer formation at higher temperatures, it is generally desirable to conduct the polymerization of lactams at temperatures at low as possible. Low polymerization temperatures are also advantageous for other obvious reasons such as simplifying operational techniques and avoiding oxidation of the polyamide.
It has been recognized in the prior art that the speed with which an anionic polymerization will proceed is generally dependent upon the type and amount of catalyst used, the type and kind of initiator used, the type of lactams being polymerized, and the temperature at which the polymerization is conducted. Of these variables, the two most significant ones, with respect to any given polymerization, are the selection of the initiator species and the selection of the temperature of the reaction. It is generally desired to conduct a rapid polymerization at low temperatures significantly below the melting point of the polymer since the polymerization proceeds to a greater extent at lower temperatures and, therefore, the selection of the initiator species becomes a controlling consideration.
Generally, it has been suggested by the prior art that certain initiator compounds may be categorized as "slow" and other as "fast". By this is meant that it has been observed that the so-called slow initiators will not cause the polymerization reaction to proceed until the temperature has been raised substantially above those temperature levels required by the so-called fast initiators. Since the initiator compound ultimately forms at least one end of the polylactam chain, the end groups on the polymeric chain are dependent upon the initiator species that are selected. From this it can be understood that the selection of end groups in the present state of the art is limited, as a practical matter, to those derived from the so-called fast initiators. Conversely, if the desired end group forms a part of a so-called slow initiator, undesirably high polymerization temperatures may be required. In still other cases, the desired end group cannot usefully be included in the initiator compound at all, due to the fact that an initiator so formed will not serve to initiate a useful polymerization reaction even at elevated temperatures well beyond the melting point of the polylactam. It has now been found that by the use of certain novel initiator compounds, the hindering effect of the end groups can largely be overcome, making it possible to prepare polylactams by anionic polymerization processes, having heretofore unobtainable end groups.